A new concept for a general class of receptor affinity biosensor is proposed. The sensors are designed to measure the concentration of ligand based on the change in surface charge associated with activation of membrane embedded receptor molecules induced by formation of ligand- receptor complex. The biosensor has general applicability based on the hypothesis that displacement of charges from activated receptors is a common phenomenon associated with changes in conformation of proteins. This is known to be the case for the nicotinic acetylcholine receptor and is likely to be applicable to a broad class of membrane bound proteinaceous receptor molecules. The biosensor may be capable of measuring ligand-receptor binding of many receptors, even those which are associated with second messenger and enzyme systems. As a research tool, the biosensor would promote study of environmental parameters which alter binding, structure activity relationship, and evolution of receptors. Pharmaceutical companies could use an array of affinity biosensors as a rapid, economical means of screening new drugs. In medicine, forensics, and law enforcement they could be used to measure the concentration of a variety of drugs in biological fluids. Concentration of product or substrate could be monitored to regulate tissue cultures, fermentors and bioreactors.